1. Field of the Invention
The invention relates to an optical near-field generating apparatus generating an optical near-field by illuminating a light to a scatterer composed of a conductive material, a method of generating an optical near-field, and an information recording and reproducing apparatus.
2. Description of the Related Art
Recently, as a method of forming a minute optical spot exceeding a diffraction limit of a light, use of a local electromagnetic field called an optical near-field is drawing attention. For example, in the field of an information recording apparatus, heat assisting electromagnetic recording using a minute optical spot formed by using the optical near-field is getting attention as a promising technology for next-generation high-density magnetic recording. This heat assisting electromagnetic recording technology enables magnetic recording to a magnetic recording medium resistant to heat fluctuation and having a high coercivity. Specifically, a light is condensed to the surface of a magnetic recording medium to locally raise the temperature of the magnetic recording medium. In the portion of the magnetic recording medium where the temperature has been raised, the coercive force of the magnetic recording medium decreases and thereby magnetic recording with a standard magnetic head becomes possible. To attain high-density magnetic recording, the size of a condensed optical spot needs to be made smaller, and a technology using the optical near-field exceeding a diffraction limit of a light has been devised. As a method of realizing a minute condensed optical spot using the optical near-field, a method using a surface plasmon resonance of a metal scatterer is available, and various studies are being made because the structure of the scatterer has a great effect on the condensing efficiency and the optical spot size of a light.
Referring to FIG. 1, description is made with respect to an example of a method of realizing a minute condensed optical spot using the surface plasmon resonance of a metal scatterer. As illustrated in FIG. 1, on a flat surface of a substrate 401, which is generally made of an optically transparent material, a scatterer 410 made of a conductive metal in a rod-like shape is formed. A surface plasmon can be excited on the scatterer 410 by arranging the scatterer 410 such that the longitudinal direction of the scatterer 410 and the polarizing direction of a propagating light Li illuminated to the scatter 410 agree with each other and by appropriately selecting the longitudinal length of the scatterer 410 to meet conditions for causing a surface plasmon to be excited.
If the propagating light Li is illuminated from the side of the substrate 401 to the scatterer 410 arranged to meet the appropriate conditions as described above, as illustrated in FIG. 2 which is a section diagram at a broken line in FIG. 1, in a light reception surface 410d of the scatterer 410, to which the propagating light Li is illuminated, and in a light emerging surface 410e, which is on the opposite side of the light reception surface 410d and which opposes an illuminated body 450 to which an optical near-field is illuminated, a charge bias is caused by an electric field of the incident propagating light Li. The oscillation of this charge bias is the surface plasmon, and if the resonant wavelength of the surface plasmon and the wavelength of the incident light Li agree with each other, it becomes a resonant condition called the surface plasmon resonance, and the scatterer 410 becomes an electric dipole strongly polarized in the direction corresponding to the polarizing direction of the incident propagating light Li, indicated by an arrow P in FIG. 2. Then, large electromagnetic fields are generated in the vicinity of both ends in the longitudinal direction of the scatterer 410, and an optical near-field Ln is generated. As illustrated in FIG. 2, the optical near-field Ln is generated in both of the light reception surface 410d and the light emitting surface 410e of the scatterer 410, however, respective optimum resonance wavelengths are different depending on the materials and the shapes of surrounding structures. When illumination of an optical near-field to the illuminated body 450 such as an information recording/reproducing medium, etc. is considered, the shapes of the surrounding structures may be adjusted so that the optical near-field in the light reception surface 410e is stronger.
By adopting the above-described method, an optical near-field with a minute optical spot can be generated from a propagating light; however, it is desirable that the conversion efficiency from the propagating light to the optical near-field is high. This is because that if the conversion efficiency is high, the power of a light emitting source such an LD, etc. that may be required for obtaining a desired power of the optical near-field can be suppressed to contribute to decreasing the power consumption and size of an optical near-field generating apparatus. Further, when the light of the light emitting source is condensed with a condensing element to be illuminated, it is possible to use a condensing element with a relatively low numeric aperture, and optical adjustment becomes considerably simple as compared with a case that a condensing element with a relatively high numeric aperture may be required, so that the yield of apparatuses can be raised.
To obtain high conversion efficiency, Japanese Unexamined Patent Application Publication No. 2003-114184, for example, discloses technology to make a scatterer in such a shape that the width of the scatterer is decreased toward a tip end part thereof where an optical near-field is generated, e.g., a plane triangular shape. The application also discloses technology to use two scatterers each in such a shape that the width decreases and to arrange the scatterers such that the narrowed tip end parts thereof come close to each other to further increase the optical near-field to be generated.